Removal of humidity from granules of plastics materials must be carried out because during transformation or polymerisation the granules are brought to fusion at relatively high temperatures and any water content in them becomes a component in the polymer molecular chain which results in breaking the polymeric chains, and in bubbles, blowholes and superficial defects being generated in the polymerized material, thus affecting the mechanical features thereof.
Many dehumidification processes for granules of plastics materials have been proposed so far. A mostly adopted dehumidification process provides the use of dry air, i. e. dried air is caused to flow through a stock of granules in order to remove any humidity (water) therefrom.
A dehumidification apparatus for granules of plastics material typically comprises a hopper arranged to contain granules of plastics material to be dehumidified, and a drying machine usually referred to as “dryer” designed to supply hot and dried air. In the case of small production plants, and thus where low air flow-rates are involved, hot compressed air is used as drying fluid.
Humidity-saturated air at a given temperature and pressure has a greater humidity content than saturated air at a lower pressure. When compressed air is expanded its humidity content is diluted in its expanded volume, and thus its relative humidity drastically drops to dew point values ranging from −15° C. to −25° C. The real value of humidity depends upon compression pressure and features of the condensation separation devices provided in a compressed air production plant. The condensation otherwise would flow in the ducts.
When a compressor is in operation, it sucks air from the environment in order to bring it to a predetermined pressure while heating it by compression. When air becomes cooled at ambient temperature, it releases the humidity exceeding that compatible with the temperature it has reached, whereby giving rise to condensation. In order to prevent condensation from reaching the air feeding line filters and traps must be provided in the feeding line. In some cases, use is made further humidity damping systems including cooling batteries or adsorption dehumidifiers.
Conventional small dehumidifiers have been developed for plastics materials in which use is made of compressed air as a means for heating and removing water content therefrom. Use of compressed air is not convenient beyond given machine dimensions as compressed air is highly expensive in terms consumed energy, and thus beyond certain rather strict limits other technical solutions such as those based on conventional dehumidifiers provided with a blower on board becomes more convenient. In practice, compressed-air dehumidifiers with a capacity higher than 30 liters are no longer competitive with respect to a conventional system.
FIG. 1 shows a conventional compressed-air dehumidifier. Compressed air at a pressure ranging from 6 to 8 bar is fed by means of a duct A controlled by a main electrovalve B which opens when the dehumidifier is activated. A pressure reducing valve C controls the pressure of the compressed air in order to obtain a desired flow rate. Air flow rate is then divided into two parallel branches of the circuit: one branch being intercepted by a flow regulator E and the other by an electro-valve D. The main portion of the flow passes through the electro-valve D and reaches a fixed narrowing F designed to avoid an excessive flow rate. After a partial expansion the air reaches a heating chamber G in which it is heated to a desired temperature and then fed through a diffuser cone I located in a lower zone of a hopper H in which the granular material to be dehumidified is located. Hot air coming out from the diffuser cone I flows from the bottom to the top of the hopper H throughout the granulated material L located in the hopper H. A temperature sensor M at the upper part of the hopper or in the air discharge ducts detects the temperature of the air discharged from the hopper. When an air temperature is detected that is higher than a predetermined temperature value the electro-valve D is operated in order to reduce the air flow rate to the heating chamber G. In this way, once the granular material has been heated up to a predetermined temperature, only a minimum flow rate of dried air is maintained through it, so as to ensure a suitable dehumidification and temperature level throughout the dehumidification process.
Many variations exist to the above described circuit, sometimes for the same purposes some other times for improvement purposes, with different configurations of the various components but operating in the same way as the above described apparatus. More particularly, an interesting solution is disclosed in EP-0 995 959, where use is made of a unit of five electro-valves in place of the electro-valve D and the flow regulator E. Each electro-valve has a through light of different size, so that by selecting the various valves different flow rates can be obtained. A step control device is thus the circuit is thus available designed to control the flow rate as a function of the characteristic features of the material to be processed.